Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $239.56   11.420%   $266.9178   $266.92   $266.90   $266.90  
  Purchase #2 $166.25   6.130%   $176.4411   $176.44   $176.45   $176.40  
  Purchase #3 $33.49   7.484%   $35.9964   $36.00   $36.00   $36.00  
  Purchase #4 $238.89   9.399%   $261.3433   $261.34   $261.35   $261.30  
  Purchase #5 $219.36   10.131%   $241.5834   $241.58   $241.60   $241.60  
  Purchase #6 $248.55   6.221%   $264.0123   $264.01   $264.00   $264.00  
  Purchase #7 $83.55   8.285%   $90.4721   $90.47   $90.45   $90.50  
  7 purchase totals = $1,229.65       $1,336.7664   $1,336.76   $1,336.75   $1,336.70